1. Field of the Invention
The present invention relates to an input protection circuit which prevents an excessive voltage such as a static charge from being applied to the gate of a MIS (Metal Insulator Semiconductor) type transistor provided on the surface of a semiconductor substrate.
2. Description of the Prior Art
In a semiconductor integrated circuit comprising a MIS transistor, the gate of the MIS transistor is connected to an input terminal. The insulating film between the gate electrode of the MIS transistor and the substrate is very thin. Therefore, when an excessive voltage is applied to the gate electrode from the input terminal, gate breakdown, for example, a short-circuit between the gate and source, occurs.
Unwanted excessive voltage is likely to be applied through handling in the manufacturing process. Static charges in particular result from handling by people.
It is impossible to realize an environment which is not occupied by static charges, and thus the integrated circuit of a MIS transistor is usually provided with an input protection circuit to be connected to the input terminal.
The ordinary input protection circuit comprises a resistor and a protection transistor. A resistor is connected between the input terminal and the gate of the MIS transistor, and the drain of the protection transistor is connected to the gate of the MIS transistor to be protected, while the source is connected to the ground line, respectively. This protection transistor is a field transistor having a thicker gate insulation film than that of an ordinary MIS transistor and is the same as an NPN lateral transistor (Drain (N)-channel (P)-source (N)) in operation. The breakdown voltage of this protection transistor is determined by the PN junction breakdown voltage between the drain and channel.
For years, continuous efforts in semiconductor integrated circuits have been oriented to the realization of higher integration density, and this tends to make the gate insulation film of the MIS transistor thinner, resulting in lower gate breakdown voltages. If the gate breakdown voltage becomes lower than that (PN junction breakdown voltage) of the protection transistor, the protection function of the conventional protection transistor is no longer effective.